Liquid crystal display devices have been used broadly as monitors for various information apparatus such as television receivers or personal computers due to their properties of low profile and low power consumption. In recent years, a drive circuit has been formed on a substrate out of thin-film transistors (hereinafter also referred to as “TFT”) using a low-temperature polycrystalline silicon (Si) film so as to achieve high definition and low cost. According to the low-temperature polycrystalline Si technology, a comparatively low-priced glass substrate can be used as the substrate. When the size of the glass substrate is increased, the productivity can be improved. The process temperature of low-temperature polycrystalline Si is usually not higher than 600° C. The strain point indicating the heat resistance of the substrate is required to be higher than the process temperature. Therefore, a glass substrate having a strain point at about 700° C. is used. A quartz substrate having a higher strain point of about 1,000° C. can be used. However, the quartz substrate is expensive and difficult to be formed with a large area.
A laser crystallization method in which an amorphous Si film is formed and then annealed with a laser beam so as to be crystallized is generally used as a method for manufacturing a semiconductor thin film as typified by a polycrystalline Si film. Particularly, a so-called excimer laser annealing system using a pulsed excimer laser beam capable of obtaining high laser energy is often used.
In recent years, a method in which crystals are grown laterally (in the scanning direction of a laser) using a so-called CW (Continuous Wave) laser beam or a pseudo-CW laser beam with an extremely high pulse repetition frequency not lower than several tens of MHz has been researched as a method for forming a semiconductor thin film having higher mobility than that of a semiconductor thin film obtained by an excimer laser annealing process. However, there is a problem that the output of each of the CW laser beam and the pseudo-CW laser beam is lower than that of the pulsed excimer laser beam so that the throughput is low.
JP-A-2004-56058 discloses a SELAX (Selectively Enlarging Laser Crystallization) method in which only a peripheral circuit portion requiring especially high-performance TFTs is irradiated with a pseudo-CW laser beam so as to be selectively provided with high performance. JP-A-2003-086505 discloses a method in which an Si film formed like islands is irradiated with a CW laser beam so that high-performance TFTs are formed.
For example, a CW laser beam obtained by converting the wavelength of a solid-state laser beam of 1,064 nm into 532 nm is used as the CW laser beam. There is a method in which the beam is shaped like a so-called line which will be a rectangle longer in the vertical direction than in the scanning direction on a substrate in order to expand the area which can be processed at one time, and to improve the throughput.
In order to improve the throughput, it is advantageous that a plurality of regions of semiconductor devices serving as circuits are disposed in a straight line on a substrate and irradiated with a CW laser beam while the substrate is scanned therewith so that the plurality of regions of the semiconductor devices are crystallized in one scan. It was, however, proved that when a glass substrate (hereinafter also referred to as “substrate” simply) is cut, cracks about several μm deep begin at the cutting position and propagate to the laser-irradiated areas on the substrate surface. FIG. 19 is an explanatory view of the method for crystallization using a CW laser. FIG. 20 is a view showing the stress generated by irradiation with the CW laser beam and the cutting position of the substrate. FIG. 21 is a view for explaining the state where cracks propagate due to the cutting of the substrate.
In FIG. 19, a substrate GLS is irradiated with a CW laser beam CWB while being scanned therewith in a short beam width direction S of the CW laser beam CWB. The CW laser beam CWB has a linear shape whose short beam width SBW is extremely shorter than the longer beam width LBW. Due to this irradiation, crystals of a semiconductor film PSI formed on the substrate GLS and made of polycrystalline Si are grown laterally. Thus, a semiconductor film SLX is obtained. When the region of the semiconductor film SLX formed by the irradiation with the CW laser beam was cut along a cutting position CUT shown in FIG. 20, it was observed that minute cracks CLK began at the cutting position CUT and propagated across the laser-irradiated region on the substrate surface, as shown in FIG. 21. When such cracks CLK are generated, there occurs a failure such as disconnection in a device formed on the substrate GLS and made of a thin film.